1. Field of the Invention
This invention relates generally to a steering apparatus for use in a motor vehicle. More particularly, the present invention relates to an electrically assisted power steering apparatus for providing assistive torque for steering a motor vehicle.
2. Disclosure Information
Electrically assisted steering systems are well known in the art. The primary function of any power assist steering device is to provide a predetermined desired assistive torque or force to the steering mechanism to reduce the effort required by the operator. Hydraulic systems accomplish this task using a network of complex hydraulic circuits to achieve a desirable boost curve. The boost curve relates the amount of assist torque that is to be generated for a given operator input torque. This curve is generally non-linear, providing low gain when the operator applies low torque, and with the gain increasing substantially as the operator applies more and more torque. Essentially, this results in low assist by the power steering system during high speed operation, allowing for what is characterized as good steering feel. Generally, it is difficult for an operator to generate high input torque while the vehicle travels at higher speeds, as vehicles are generally very responsive to steering input when at speed.
On the other hand, it is quite common for an operator to generate considerable steering torque during low speed operation, such as parking. This is when the system must be capable of providing substantial assistive torque, in response to the high operator input torque. The typical boost curve reflects this by providing increasing gain as the steering input torque increases. An electrically assisted power steering system must perform this same function.
The performance capabilities of an electrically assisted power steering system may be characterized by its bandwidth, which is a measure of the system response time and system gain. Power assist steering systems must have sufficient bandwidth to respond seamlessly to the driver's fastest inputs while at the same time preserving the feel of the road through the mechanical steering mechanism. If dynamic steering frequency exceeds the system bandwidth, the operator will detect a degradation of the steering feel, the feel becoming "sluggish" as the electric motor can not generate the demanded assist torque. To avoid this, the system must have sufficient bandwidth while maintaining the stability and system vibrations within predetermined acceptable levels. In hydraulic power assist systems, hydraulic stability and vibrations could be controlled by lowering assist gain or by increasing damping. However, because increased fuel economy is one of the objectives of using an electrically assisted power steering system, it is desirable to seek alternatives to adding damping to control the stability and vibrations.
It would therefore be desirable to develop an electrically assisted power steering system having sufficient bandwidth to satisfy the demands of a power steering system while maintaining control system stability and vibrations within predetermined levels without employing excessive damping, thereby enhancing the fuel efficiency benefits attainable with electrically assisted power steering systems. Additionally, it would be desirable to develop a system capable of using low cost sensors and actuators without overburdening the microprocessor.